1. Field of the Invention
This invention relates to an automatic transmission for a vehicle, and more particularly to a power train of an automatic transmission for a vehicle which can increase fuel efficiency by improving an efficiency of power transmission in a high speed range, and can increase a feeling of smooth transmission by continuously being changed a low speed range.
2. Description of Related Art
Generally, an automatic transmission includes a transmission control unit which automatically controls the ratio of speed change according to a speed of the vehicle and load applied to the vehicle.
The transmission control unit controls the RPM of an output end of a planet gear set by operating or releasing a plurality of clutches and brakes provided in the control unit.
Practically, to design a gear train which outputs forward speed and backward speed, at least one multiplanet gear set and a plurality of operating elements must be provided. And to design a gear train which smoothly changes speed, at least one multiplanet gear set, seven operating elements and three one-way clutches must be provided.
A gear train having the several elements, however, is complicated in its construction and a weight of the gear train is increased.
Since the prior automatic transmission has limited the speed changing steps, a shock occurs upon changing speed and the range for operating the engine is limited within some steps. Fuel ratio is lowered and power of the engine cannot be used in its best capacity.
Since in a low speed range, input torque is high and speed must be frequently changed and in a high speed range planet gears are not directly connected to the crank shaft of the engine, the efficiency of power transmission is decreased.
To solve the above problems, an automatic transmission having a mechanism in which a turbine of a fluid torque converter is directly connected to the input shaft of a planet gear assembly in an automatic transmission is disclosed in U.S. Pat. No. 4,776,240 issued to Aisin.
In the automatic transmission according to the Aisin patent, however, as mentioned above, seven operating elements and three one-way clutches must be provided, and changing speed is not continuously performed according to the conditions of a vehicle. Therefore the above problems are not perfectly solved.